With an increasingly serious global energy crisis, electric vehicle with advantages of saving energy and reducing pollution has drawn much attention. A power battery in an electric vehicle may store a great deal of energy. Currently, a lithium iron phosphate (LiFePO4) battery is widely adopted as the power battery in the electric vehicle due to its numerous advantages, such as high discharge rate (5-10 C), stable discharging voltage, safety in use, long service life and low environmental pollution. However, when the lithium iron phosphate battery used as an in-vehicle power battery works in a low temperature environment, an internal resistance of the battery becomes too large such that the battery is difficult to be charged.